1. Field of the Invention
The present invention relates to a method for soldering one member to another member, and more specifically to a method for soldering a land formed on a board and an electrode of electronic component (e.g., lead) in a process of manufacturing an electronic circuit board. Furthermore, the invention relates to a joining structure obtained by this soldering method and an electrical component or an electronic component used in this soldering method.
2. Description of the Related Art
In processes for manufacturing an electronic circuit board used for the electronic equipment, the method of reflow soldering has hitherto been known as one of methods used for mounting an electronic component on a board, more specifically for joining electrically and physically a lead pulled out of the electronic component to a land formed on the board.
In a general method of reflow soldering, at first, a so-called cream solder (not shown in drawings) is provided by screen printing on a land that is part of a wiring pattern formed on the board. The cream solder in general is made by mixing a solder powder comprising a soldering material with a flux comprising rosin, activating agent, and solvent. Thereafter, an electronic component is set on a prescribed portion of a board so that a lead pulled out of the electronic component adheres to the cream solder set on the land to be joined. The flux is activated by a thermal treatment of the board on which the electronic component is thus set through the cream solder at a temperature higher than the melting point of the soldering material used, thus a soldering material forming the solder powder in the cream solder is once fused, and simultaneously other components such as the flux in the cream solder is allowed to evaporate (or volatilize). Subsequently, the soldering material fused is solidified by cooling (or standing for cooling) The solidified soldering material forms a joining portion between the lead of the electronic component and the land of the board to join them electrically and physically. Although other components than the soldering material such as the flux can exist in the joining portion, these other components causes phase separation from the soldering material at the thermal treatment, fail to exist inside the joining portion, and only remain slightly on the surface of the portion. Thus, an electronic circuit board having the electronic component mounted on the board by the joining portion (or soldering portion) comprising the soldering material substantially is obtained.
Soldering materials used generally include Sn—Pb series materials, particularly that having an eutectic composition (Hereinafter simply referred to as a Sn—Pb eutectic material). It is known that the eutectic composition of the Sn—Pb series material is a Sn—37Pb composition (i.e., a composition consisting of 37 weight percent of Pb and the residual Sn (of 63 weight percent)), and the Sn—Pb series material has a melting point of 183° C. in this eutectic composition. For example, see Unexamined Japanese Patent Publication No. 2000-260801.
In recent years, the waste disposal of electronic equipment having the above-described electronic circuit board have offered a problem, and effects on the global environment and the human body exerted by the lead (Pb) contained in soldering materials have become a matter of concern. Therefore, there's movement toward use of materials containing no lead, that is, lead-free soldering materials as soldering materials in place of the Sn—Pb series materials used so far, and the practical use thereof has been attempted.
Recently, materials having various compositions have been proposed as the lead-free soldering materials and one of such materials is a Sn—Zn series material. As a result of recent studies, it was found that the Sn—Zn series material had an eutectic composition of about Sn—9Zn (i.e., nine weight percent of Zn and the residual Sn (91 weight percent)) and the Sn—Zn series material had a melting point of 199° C. in the eutectic composition.
It is desirable that the melting point of the lead-free soldering materials are sufficiently low to the extent that the electronic component is not damaged and is comparatively near to the melting point of the conventional Sn—Pb series materials in consideration of the heat-resistant temperature of the electronic component to be mounted on a board and the application of an existing soldering method. Since the melting points of the above-described Sn—Zn series materials are lower than those of other lead-free soldering materials such as Sn—Ag series materials and comparatively near to those of the Sn—Pb series materials, the Sn—Zn series materials are expected as leading substitutes for the Sn—Pb series materials.
Although use of the Sn—Zn series materials in place of the Sn—Pb series materials had an advantage of soldering an electronic component to a board without causing thermal damage to the electronic component, however, it was found that the joining portion between the land of the board and the lead of the electronic component was deteriorated and no sufficient strength of thermal fatigue resistance was obtained as a result of the continuous use test of the resulting electronic circuit board under a condition of high temperature.
This is thought to be caused by contact of lead (Zn) contained in the Sn—Zn series materials with copper (Cu) used as a material of the land and the lead and forming an intermetallic compound comprising Cu and Zn at the joining interfaces between the joining portion and the land and between the joining portion and the lead.
An electronic circuit board prepared by soldering an electronic component and a board by use of a cream solder containing cream powder comprising the Sn—Zn series material in place of the Sn—Pb series material according to the above-described conventional method of reflow soldering is hereinafter illustrated through FIG. 2.
In this electronic circuit board, a land 7 formed on a board 6 and a lead 9 pulled out of an electronic component 10 are joined electrically and mechanically. The land 7 in general is made of Cu and formed in one united body with a wiring pattern. Furthermore, the lead 9 in general is prepared by covering a parent material 9a made of Cu with a plating 9b comprising a Sn—Pb eutectic material. A joining portion 8 is formed by a thermal treatment of the cream solder, and formed substantially of a soldering material stemming from the solder powder as described above.
Although freedom from lead is promoted also as to a soldering material for the lead of an electronic component at present, the Sn—Pb series materials are still used nowadays as a transition period in some cases.
Since the soldering material directly contacts with the land 7 in the thermal treatment, Cu forming the land diffuses into the soldering material and combines with Zn to form an intermetallic compound 11 comprising Cu and Zn at this joining interface.
Furthermore, as the soldering material of solder powder fuses in the thermal treatment, the plating 9b consisting of a Sn—Pb eutectic material having a melting point lower than the temperature of the thermal treatment also fuses, the portion of the plating 9b contacting with the soldering material in a fused condition fuses and diffuses into the soldering material. Consequently, the plating 9b of the lead 9 is peeled off at places and the parent material 9a comes to contact directly with the soldering material in the fused condition. Accordingly, the Cu composing the parent material 9a of the lead 9 diffuses into the soldering material similarly as described above and combines with Zn to form an intermetallic compound 12 comprising Cu and Zn at the joining interface.
When the board having the electronic component soldered as described above are placed particularly under a condition of high temperature for a long period of time, more much Cu diffuses into the joining portion (or the soldering material) to promote the formation of the intermetallic compounds 11 and 12 comprising Cu and Zn. All Zn in the soldering material is soon consumed for the formation of the intermetallic compound comprising Cu and Zn, subsequently Cu remaining without contributing to the formation of the intermetallic compound and/or Cu composing the Cu—Zn intermetallic compound diffuse into the soldering material, and spreading diffusion of Sn in the soldering material into voids produced by the diffusion of the Cu, that is, interdiffusion occurs. This phenomenon is thought to invite the deterioration of the joining portion.
Also, in the case where a cream solder containing the solder powder comprising the Sn—Pb series material is conventionally used, the copper (Cu) used as a material for the land and/or the lead diffuses into the joining portion and combines with tin (Sn) contained in the Sn—Pb series material, and thereby the intermetallic compound comprising Sn and Cu can be formed between the joining interfaces and the land and/or the lead at the joining portion. However, since the intermetallic compound comprising Sn and Cu is stable, the compound can endure continuous use under a condition of high temperature. Accordingly, the formation of the intermetallic compound comprising Sn and Cu is thought to invite no problem of deterioration of the joining portion that occurs in the case of the intermetallic compound comprising Cu and Zn.
Furthermore, a Fe—42Ni alloy material (i.e., an alloy material having a composition comprising 42 weight percent of Ni and the residual Fe (58 weight percent)) is used in some cases as the material for the parent material 9a of the lead 9 as well as Cu. In this case, the intermetallic compound 12 comprising Cu and Zn is not formed at the interface between the lead 9 and the joining portion 8 by allowing the plating 9b on the lead 9 to fuse and diffuse into the soldering material and allowing the parent material 9a to contact directly with the soldering material. However, since the intermetallic compound 12 comprising Cu and Zn is still formed at the interface between the land 7 comprising Cu and the joining portion 8, deterioration of the joining portion occurs when the resulting electronic circuit board is subjected to a continuous use test under a condition of high temperature. Accordingly, this alloy material also has the problem of having no sufficient strength of thermal fatigue resistance.
Although the Sn—Zn series material has been described above in detail, other soldering materials containing at least Sn and Zn also encounters similar problems.